This invention is based on an apparatus for active motor vehicle suspensions and relates to active vehicle suspensions in motor vehicles; concepts based on known load leveling systems are presented that, taking these load leveling systems as a point of departure, illustrate specialized aspects of the invention.
For instance, it is known that conventional load leveling suspension systems correct changes in the position of the vehicle body with respect to the roadway that are due solely to static loads, and that they require very little energy for making such corrections. On the other hand, fully active suspension systems having a fast servomechanism between the wheels and the body must be capable of reacting in the frequency range of the natural wheel motion and therefore require an extremely large amount of energy, in order to isolate the vehicle body from the interference originating at the wheels.
A further class of active systems, which react actively only to low-frequency inputs (natural vehicle body frequency), but not to high-frequency inputs, could be characterized as a fast-acting load leveler suspension (FLLS), or fast load-change compensation system; the term "load change" is intended to mean a supplementary force (centrifugal force) resulting from vehicle maneuvers (cornering, braking) or from the vehicle load.
In designing a motor vehicle suspension system, such a system must also include passive spring and damper elements; the springs can be integrated in the active leveling units (air suspension; see FIG. 1) or, in the conventional manner, disposed parallel to the damper (hydraulic leveling unit; see FIG. 2), and the damper element may either be passive, active, or semi-active. The variants shown in FIGS. 1 and 2 for the topological arrangement of suspension elements (leveling unit, spring, damper) do have gradual, but not basically different, effects on the behavior of the system.
Further discussions of this general field and of known systems can be found in the following sources: Dean Karnopp, "Active Damping in Road Vehicle Suspension Systems", Vehicle System Dynamics, Vol. 12, No. 6, 291-311; Bruce MacLaurin, "Progress in British Tracked Vehicle Suspension Systems", SAE Paper No. 830442, 1983; Dean Karnopp, "Two Contrasting Versions of the Optimal Active Vehicle Suspension", Dynamic Systems: Modeling and Control, ASME Publication DSC-Vol. 1, 1985, 341-346, and finally, the published European Patent Application corresponding to EP-A 0196316. In these sources, semi-active damping systems for damping the courses of motion of two bodies, in particular in a vehicle wheel/body suspension system, are explained in detail.
On the basis of a fast-acting load leveler suspension as shown in FIG. 1 or FIG. 2, in combination with passive damping elements, the following basic observations can be made, and the problems associated with such a system can be identified:
If the active load leveling components are incorporated into the regulation of the spring deflection, then at high gain values, an attempt at fast compensation of static deflections causes vehicle body instability. Low gain factors (slow leveling) are unable to compensate for fast load changes.
In both basic systems, the FLLS brings about a reduction in the damping property of the passive damper, and thus leads to unstable system behavior, if the basic damping by this passive element is too low. This fact would necessitate either high passive damping (a hard ride), error recognition in the event of increasing instability, or an adaptation of the gain with a worsening of the system damping.
One positive aspect of an active suspension with FLLS is its "fail-safe" property, because even with the FLLS switched off, chassis function is assured by the existing passive components.
An active control system having FLLS is intended especially to control low-frequency vehicle body motion based on the above-described load change. Thus, it appears useful not to take high-frequency motion (natural wheel dynamics) into account in the regulation, that is, the closed loop control; this could be achieved by low-pass filtering of the suspension deflection, but the effects of this filtering upon overall system behavior would have to be taken into account.
Possible ways of reducing the energy requirement of such systems are the aforementioned filtering of corresponding signals and the resultant slow adjustment conditions; defined adaptation of the gain depending on the ride (high during braking; otherwise low), or an FLLS that can be switched off and is activated only as needed.
These aspects, which for the sake of better comprehension will be referred to again in further detail in the course of the disclosure, along with mathematical observations, are the basis for the present invention, for which the point of departure is such that a fast-acting load leveler suspension system having passive spring/damper components is in a position to compensate for low-frequency body movements, while high-frequency disturbances can be controlled by conventional passive components. However, this dictates a compromise in the damper characteristic, in order to avoid system instabilities. A high passive damping would therefore be absolutely necessary, which on the other hand means some sacrifices in terms of the comfort of the ride.